1. Field of the Invention
This invention relates to a radiographic intensifying screen, and more particularly to a radiographic intensifying screen having a colored protective layer.
2. Description of the Prior Art
Radiographic intensifying screens (hereinafter referred to simply as "intensifying screen") are used in face contact with radiographic films to increase the sensitivity of the radiographing system in various kinds of fields such as medical radiography for medical diagnosis and industrial radiography for non-destructive inspection of industrial materials. The intensifying screen is essentially composed of a support and a fluorescent layer formed thereon. The fluorescent layer is composed of a phosphor which emits light of high luminance under excitation of radiation and a resinous binder in which the phosphor is dispersed. The surface of the fluorescent layer is usually covered with a thin transparent protective layer as of polyethylene terephthalate, cellulose acetate, polymethacrylate and nitro cellulose.
Some intensifying screens have a reflective layer or an absorptive layer between the support and the fluorescent layer. Further, some intensifying screens used for nondestructive inspection of industrial materials have a metallic foil between the support and the fluorescent layer.
Recently, it has been strongly desired to decrease the dose of patients in medical radiography by enhancing the sensitivity of the radiographic film-intensifying screen system. Thus, a high sensitivity system employing a high sensitivity intensifying screen has been developed and put into practical use. The high sensitivity intensifying screen contains a phosphor having high X-ray absorption and high X-ray to light conversion efficiency such as terbium activated rare earth oxysulfide phosphor. In the intensifying screen, it is desirable that the sensitivity to radiation or X-ray to light conversion efficiency be high. Beside this, it is also desirable that the photographic quality of the obtained image such as sharpness and granularity be improved.
It has been known in the art to enhance the sharpness of the image in radiography by employing a colored protective layer on the fluorescent layer of an intensifying screen as disclosed in U.S. Pat. No. 4,012,637. In this patent, it is disclosed that the sharpness is enhanced by decreasing the amount of fluorescent light advancing in the direction not perpendicular to the surface of the intensifying screen.
On the other hand, the granularity known to be increased by the statistic fluctuation of X-ray quanta accompanying decrease in dose of incident X-ray used for radiographing is difficult to improve in the high sensitivity system. In the high sensitivity system employing a phosphor having high X-ray to light conversion efficiency, the granularity is markedly increased or degraded although the sensitivity of the intensifying screen is enhanced. Accordingly, particularly in the high sensitivity system employing the intensifying screen containing a phosphor of high X-ray to light conversion efficiency, it is highly desired that the granularity be improved to enhance the quality of the radiographic image obtained.